The present invention relates to the preparation of a lithographic printing plate by means of direct ink jet printing.
Traditional techniques of printing include letterpress printing, gravure printing and offset lithography. All of these printing methods require a plate, usually loaded onto a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing cylinders, in contrast, contain series of wells or indentations that accept ink for deposit onto the recording medium.
In the case of traditional offset lithography the image to be printed is present on a plate as a pattern of ink accepting (oleophilic) areas on an ink repellent (oleophobic or hydrophilic) background. In the wet system the required ink repellency is provided by an initial application of a dampening (or fountain) solution prior to inking. Conventional presensitized lithographic printing plates bear a UV sensitive coating based on photopolymer or diazonium chemistry. The plates have to be UV-exposed through a mask carrying the image. The mask is a graphic arts film prepared by photographic techniques based on silver halide chemistry and involving exposure by a camera or by an image-setter, and further involving wet processing. Depending on the type of plate used (negative or positive) the non-image areas or the image areas are solubilized thereby differentiating the plate into oleophilic and hydrophilic areas. A disadvantage of this traditional method is the necessary cumbersome preparation of the photographic intermediate film involving wet processing. A further drawback is the wet processing of the printing plate itself.
With the advent of the computer in revolutionizing the graphics design process leading to printing, there have been extensive efforts to develop a convenient and inexpensive computer-to-plate system wherein a photographic intermediate is no longer required. In recent years some of these systems are introduced into the market based on different chemical systems and exposure methods. For instance, the SETPRINT material, trade mark of Agfa-Gevaert N.V. is based on silver halide DTR chemistry and consists of a polyethylene terephthalate base carrying a photographic coating which after photo-mode exposure and processing produces complementary oleophilic and hydrophilic areas. Another system based on photo-mode exposure but with a hydrophilic aluminum base is LITHOSTAR, trade mark of Agfa-Gevaert N.V. A system based on heat mode exposure by means of an intense infra-red laser is called THERMOSTAR, also a trade mark of Agfa-Gevaert N.V.
Many of the new computer-to-plate systems are large, complex, and expensive. They are designed for use by large printing companies as a means to streamline the prepress process of their printing operations and to take advantage of the rapid exchange and response to the digital information of graphics designs provided by their customers. There remains a strong need for an economical and efficient computer-to-plate system for the many smaller printers who utilize lithographic printing.
A number of electronic, non-impact printing systems have been investigated for use in making lithographic printing plates to satisfy the needs of these smaller printers. Foremost among these have been laser printing systems, for example as described in U.S. Pat. No. 5,304,443 and references therein. Another non-impact printing system which has received attention for economical and convenient computer-to-plate preparation for lithographic printing is thermal transfer printing, for example, as described in U.S. Pat. No. 4,958,564.
In recent years, ink jet printers have replaced laser printers as the most popular hard copy output printers for computers. Some of the competitive advantages of ink jet printers are low cost and reliability. The ink jet printing system is a relatively rapid image output system and has a simple construction because it does not require any complex optical system. In recent times, there have been some reports in the literature proposing the use of ink jet printers to make lithographic printing plates.
In Japanese Kokai 62-25081, an oleophilic liquid or fluid ink was printed by ink jet printing onto a hydrophilic aluminum surface of a lithographic printing plate. Titanate or silane coupling agents were present in the ink.
An ink jet printing apparatus to make lithographic printing plates is described in PCT WO 94/11191. It is directed to depositing hydrophobic or hydrophilic substances on hydrophobic printing plates.
In U.S. Pat. No. 5,501,150, a fluid ink and hydrophilic media set containing materials to produce a silver-reducible image by ink jet printing are used to make a metallic silver image which, following wet processing to make the silver image sufficiently hydrophobic, is said to provide a lithographic printing plate.
Ink jet printing wherein the ink is a solid or phase change type ink instead of a liquid or fluid type ink is described in U.S. Pat. No. 4,833,486 to deposit a hot wax on a surface of an offset plate. Upon cooling of the wax, it solidifies, thereby providing a printing plate. Solid ink jet printing has serious disadvantages for lithographic plates in that the wax or resin image has limited durability due to its thermoplastic, chemical, and adhesive properties and the amount and rounded shape of the solidified ink jet droplet on the media do not have the intrinsic image resolution properties found in liquid ink jet printing.
There is also prior art in the use of ink jet printing to apply an opaque image or mask pattern to a photosensitive lithographic printing plate blank, as for example, in Japanese Kokai 63-109,052. The blank is then exposed through the ink jet imaged mask pattern and then processed by conventional means to provide a lithographic printing plate. This approach retains the materials and processing of conventional lithographic printing plates and only uses ink jet printing as an alternative for the photomask through which the conventional plates are exposed. Thus this approach adds to the complexity and expense of the platemaking process and does not depend on the ink jet ink image for the hydrophobic image of the plate. U.S. Pat. No. 5,495,803 describes a solid or phase change type of ink jet printing to form a photomask for a printing plate.
As a further example of the methods for preparing printing plates by using the ink jet printing system, Japanese Kokai Publication 113456/1981 proposes methods for preparing printing plates whereby ink-repelling materials (e.g. curable silicones) are printed on a printing plate by ink jet printing. The printing plate obtained by this method is an intaglio printing plate in which the ink-repelling material formed on the surface of the substrate serves as a non-image part. As a result, the resolution of the printed images at shadow area or reversed lines is not so good. Moreover, a large amount of ink is needed in this method because the ink-repelling material must be deposited on the whole non-image part which occupies most of the surface of the printing plate, thereby delaying the printing process.
U.S. Pat. No. 5,511,477 discloses a method for the production of photopolymeric relief-type printing plates comprising: forming a positive or a negative image on a substrate by ink jet printing with a photopolymeric ink composition, optionally preheated to a temperature of about 30xc2x0-260xc2x0 C., and subjecting the resulting printed substrate to UV radiation, thereby curing said ink composition forming said image. This is an obnoxious method due to the sometimes high vapour pressure and toxicity of said inks.
U.S. Pat. No. 5,312,654 discloses a method for making lithographic printing plates comprising: forming an image on a substrate having an ink absorbing layer and a hydrophilized layer between the substrate and the absorbing layer by ink jet printing using a photopolymerizable ink composition, and exposing it to an actinic light in the wavelength region with which said ink composition is sensitized to cure the image. The printing endurance of said printing plates is low.
Japanese Kokai Publication 69244/1992 discloses a method for making printing plates comprising the steps of forming a printed image on a recording material subjected to a hydrophilic treatment by ink jet printing using a hydrophobic ink containing photocurable components, and exposing the whole surface to actinic light. However, the surface of the substrate to be used for the lithographic plate is usually subjected to various treatments such as a mechanical graining, an anodizing or a hydrophilic treatment to obtain good hydrophilic property and water retention property. Therefore, even the use of an ink composition having a very high surface tension results in a poor image on the surface of the substrate because of ink spreading and low printing endurance.
EP-A-533 168 discloses a method for avoiding said ink spreading by coating the lithographic base with an ink absorbing layer which is removed after ink printing. This is an uneconomical and cumbersome method.
Research Disclosure 289118 of May 1988 discloses a method for making printing plates with the use of an ink jet wherein the ink is a hydrophobic polymer latex. However said printing plates have a bad ink acceptance and a low printing endurance.
EP-A-003 789 discloses a process for the preparation of offset printing plates by means of an ink jet method with oleophilic inks. There is not indicated how said inks are made but from the examples it is clear that it concerns artificial latices, which are difficult to prepare.
JN-57/038142 discloses a method of preparing a printing plate by forming an ink image on a blank printing plate, and also by fixing this image thermally by making toner to adhere to this image-formed area. The composition of the ink is not mentioned, only the composition of the toners is disclosed.
JN-07/108667 discloses a plate-making method forming an ink image containing a hydrophilic substance on a conductive support whose surface layer is made hydrophilic according to an electrostatic attraction type ink set system to dry or cure the same, by applying bias voltage to the conductive support at the time of ink jet writing. This is a cumbersome process.
U.S. Pat. No. 5,213,041 discloses a method for preparing a reusable printing plate for printing, projecting an imaging deposit on the plate surface by jet printing using an ejectable substance containing a heat fusible component. The image forms an imaging deposit which is fused to the surface of the printing plate using a variable frequency and variable power induction heater.
According to WO 97/43122 a lithographic printing plate is manufactured by means of an ink jet fluid comprising reactive components selected from the group consisting of transition metal complexes and organic carbonyl compounds. In a preferred embodiment the reactive compound comprises one or more chromium complexes of an organic acid.
In WO 00/46034 a printable media is disclosed, including a substrate having a hydrophilic, porous layer on at least one surface, and an ink receptive, thermoplasic image layer adhered to the hydrophilic, porous layer, wherein the ink receptive layer contains a polymer having a low surface energy and a plurality of tertiary amine sites being at least partially neutralized with an acid. Further a method is disclosed wherein the polymer having tertiary amine sites is applied in a fluid by means of ink jet printing onto the hydrophilic porous layer. A similar fluid is disclosed in WO 00/46038.
In WO 97/39894 a heat-sensitive composition is disclosed and a method of making a lithographic printing form with it. On a lithographic base there is coated a complex of preferably a phenolic resin and a compound which forms a thermally frangible complex with the phenolic resin. This complex is less soluble in the developer solution than the uncomplexed phenolic resin. Further a laser absorbing material can be present. When the complex is image-wise heated, e.g. by high-power IR-irradiation, the complex breaks down so allowing the non-complexed phenolic resin to be dissolved in the developing solution. A positive working printing form is obtained. This teaching is further elaborated in WO 98/42507 wherein specific dissolution inhibitors are disclosed.
In EP 864420 there is provided a heat-sensitive imaging element for making positive working lithographic printing plates comprising on a lithographic base a layer comprising a polymer, soluble in an aqueous alkaline solution and an IR-sensitive top layer. Upon image-wise exposure the capacity of the aqueous alkaline solution to penetrate or solubilize the top layer is changed.
The lithographic printing plate precursors based on heat mode according to the above cited references show the drawback that they require a complicated and expensive platesetter apparatus equipped with an an expensive and short-living high power IR exposure unit. Furtheron their working mode is limited to positive working.
In European patent application appl. No. 01000150 a method is disclosed for the preparation of a negative working lithographic printing plate by means of ink jet printing. This method uses a lithographic printing plate precursor comprising a lithographic support and an image forming layer containing a polymeric binder soluble in an aqueous alkaline developer. The precursor is image-wise printed by an ink jet fluid comprising a compound capable of reducing the solubility of said binder in said aqueous alkaline developer. After development a negative working printing plate is obtained.
A disadvantage of this system and of most systems disclosed in the references cited above is the need for a processing off press, so that separate processing equipment and chemicals are required.
It is an object of the present invention to provide a method for the preparation of a lithographic printing plate whereby the plate can be developed by fountain and/or ink on press, so that no separate processing equipment or chemicals are required.
It is a further object of the present invention to provide a method for the preparation of a lithographic printing plate which has the same plate and lithographic chracteristics as a conventional plate.
It is a further object of the present invention to provide a method for the preparation of a lithographic printing plate which is uncomplicated and requires only a relatively cheap ink jet printer.
The above mentioned objects are realised by providing a method for the preparation of a lithographic printing plate comprising the following steps, in order, :
(1) mounting on a printing press a printing plate precursor comprising a lithographic support and an image forming layer, capable of being dissolved and removed by the application of fountain and/or ink on said press,
(2) prior to, or after step (1), dispensing image-wise by means of ink jet printing droplets of a fluid onto the surface of said lithographic printing plate precursor, characterized in that said fluid comprises in a solvent carrier a dissolution inhibitor compound capable of inhibiting or reducing the dissolving and removing action of fountain and/or ink on said image forming layer,
(3) drying the plate precursor to at least partially remove the solvent carrier of said fluid,
(4) applying fountain and/or ink onto the dried plate precursor, thereby removing the areas of said image forming layer, not imaged by ink jet printing, and simultaneously exposing the surface of said lithographic support, while retaining completely or at least partially the areas of said image forming layer, imaged by ink jet printing.
In a preferred embodiment the image forming layer comprises an anionically stabilized monomer.